To characterize elastic properties and geometrical parameters of individual, whole myosin molecules during their interaction with actin we sparsely adsorbed myosin molecules to nanometer-sized microspheres. Thermally driven position fluctuations of these microspheres were recorded with the three-dimensional detection scheme of the photonic force microscope. Upon binding of single myosin molecules to immobilized actin filaments in the absence of ATP, these thermally driven position fluctuations of the microspheres change significantly. From three-dimensional position fluctuations stiffness and geometrical information of the tethering molecule can be derived. Axial stiffness was found to be asymmetric, approximately 0.04 pN/nm for extension, approximately 0.004 pN/nm for compression. Observed stiffness of whole myosin molecules is much less than estimated for individual myosin heads in muscle fibers or for single-molecule studies on myosin fragments. The stiffness reported here, however, is identical to stiffness found in other single-molecule studies with full-length myosin suggesting that the source of this low stiffness is located outside the myosin head domain. Analysis of geometrical properties of tethering myosin molecules by Brownian dynamics computer simulations suggests a linker length of approximately 130 nm that is divided by a free hinge located approximately 90 nm above the substrate. This pivot location coincides with myosin's hinge region. We demonstrate the general applicability of thermal fluctuation analysis to determine elastic properties and geometrical factors of individual molecules.